1. Field of the Invention
The present invention relates to digital audio signal processing, and more particularly to a method for encoding sub information contained in a signal having the sub information and main information, a recording medium, and a method and an apparatus for reproducing a signal.
2. Description of the Related Art
In general, a digital audio signal reproduced from a digital medium such as a digital audio disk is formatted to have a digital audio signal served as main information and an area termed a subcode containing sub information such as graphic information and text information.
Herein, a signal format of a compact disk, which is a digital audio disk, will be described with reference to the drawings. As shown in FIG. 1, the signal format of the digital audio disk has a frame unit as a minimum information unit. 98 frames compose one block. One frame is composed of an audio signal 6 samples (one of which is composed of 16 bits) of Right channel and 6 samples of Left channel totalling 24-byte data and a one-byte data as a subcode. It means that one block of the subcode is composed of 98 bytes.
FIG. 2 is an explanatory view showing the format of the subcode of the digital audio disk. The first two frames of one block, S.sub.0 and S.sub.1, each of which is a 2-byte data, are used as a synchronous signal to the subcode itself. Hence, the third frame or later one, which are in total composed of 96 bytes, are used as data of the sub information. Each frame of the subcode is composed of 8 bits (1 byte). Each bit of the frame is called P, Q, R, S, T, U, V, or W channel.
The P channel is used for indicating the position of the head of a tune of an audio signal corresponding to the main information. In particular, it is used for roughly locating the head of the tune. As shown in FIG. 3, the Q channel is sectioned into the head four bits for a control code, the subsequent four bits for an address code, the tail 16 bits for a code for a Cyclic Redundancy Code (CRC) of the error detecting signal, and the remaining 72 bits for the subject data. The control code is used for copy prohibition or identification of the kind of the recording medium. The address code is used as an address code of 72-bit data of the Q channel. In particular, if the address code is "0001", the 72-bit data of the Q channel indicates the format of each movement of the tune contained in the audio data as shown in FIG. 4. Further, the address codes of "0010" and "0011" are used as maker codes. In the other area of the Q channel, no format is specified. The data area may be arbitrarily used by a user.
The R to W channels are used as user channels. On these channels, the sub information such as graphic information and text information may be recorded as mentioned above.
In the Japanese Application No. Hei 7-147742, there has been proposed a technique of recording data for enhancing the sound quality of the audio signal on the data area of the subcode to be used by the user. The content will be described below.
The digital audio disk is formatted to have an audio signal sampled at 16 bits and 44.1 kHz. This signal is used for dividing the high-quality signal, for example, the 20-bit signal sampled at 88.2 kHz into the main information having a format of the digital audio disk and the sub information. The main information is recorded on the recording area of the normal main information. The sub information is recorded in the user channel of the sub code as a signal for enhancing the quality of the sound. In reproducing the signal, the main information is synthesized with the sub information for the purpose of supplying the audio signal having a higher sound quality than that of the normal digital audio disk.
On the contrary, the typical signal reproducing apparatus for the digital audio disk is arranged to read data from a recording medium through the use of a light pick-up unit and reproducing the main information for the audio signal and the sub information recorded on the subcode through the use of a demodulating LSI. At this time, the main information is stored in a memory built for temporarily saving the digital data read from the disk and synchronized to a stable clock before it is outputted. In practice, however, the sub information recorded in the subcode is being outputted at the same speed as the reading one of the information without being synchronized to such a stable clock. Hence, the output suffers from time fluctuation on called jitter, because the uneven rotation of a motor makes the reading speed unstable. This jitter takes place in not only the inside of the signal reproducing apparatus but also the digital output of the apparatus. For example, in a case that the signal reproducing apparatus provides a terminal and a signal having a digital audio format specified by IEC 958 (EIAJ CP1201) is outputted from the terminal, the fluctuation takes place in the user bit specified thereby.
Under these circumstances, with the data for enhancing the sound quality of the audio signal, the audio data corresponding to the main information is closely related with the sub information recorded in the subcode in light of time. Hence, it is preferable to synchronize the main information with the sub information.